Toyota may not have the specs for the radio.
Chances are it's either a Pioneer or a Delco, although it may also be a
Clarion, Fujitsu Ten or National (panasonic). Most North American made
vehicles have Delco.

You'll have to pull the radio (unless so marked on the faceplate) and
contact the MFGR and see if they can help you; I bet they will say the
information is proprietary and may tell you to call Toyota.

Good luck!

Oh, BTW, my bet is that it does not have RCA outs. It will have it's own
plug system. There may be someone out there that makes an adapter for it.
If you get the make you may be able to get a connector/cable for it.

WHat you can do is trace the wiring from the sub back. Chances are there's
an amp under the seat or the console. You could make up a special cable,
install another amp and replace the sub.

Again, good luck. And let me know what you find. I was considering doing
this to a Scion, but I got a JVC and a faceplate on EBay and just replaced
the whole mess...

As one of those guys who likes Laurence Welk's poka music and gets a
LOT better gas mileage at 45 than 85 mph and who's left turn signal
light is burnt out, I have some advice for you.  Pull the radio out of
the dash and LOOK AT THE BACK.  That's assuming you know what an RCA
jack looks like.  Of course you might need someone to point out to you
the proper screw removal tool and instruct you on which way to turn
the screw.

Jack

Subs are usually monophonic except in live music venues, where they
sometimes add more not for stereo separation, but for more bass, which tends
to be relatively non-directional.

Toyo makes tires, not vehicles so you will have better luck contacting
Toyota or going to Toyota's technical web site at www.techinfo.toyota.com 
for $10/day.  The wiring diagrams for your audio system should be there.

I can tell you for free that there are no RCA inputs/outputs for an external
amplifier, if that is all you want to know.